NK-T-B antigen (NTB-A), also known as signaling lymphocyte activation molecule family member 6 (SLAMF6), CD352, Ly-108, SF2000 and KALI, is a type I transmembrane protein belonging to the SLAM family of immune cell receptors. SLAM family proteins are members of the CD2 subgroup of the Ig superfamily NTB-A is expressed on natural killer (NK), T and B cells and exhibits homotypic interactions, mediated by recruitment of SLAM associated protein (SAP) and additional adapter proteins to the receptor complex.
NTB-A contains two extracellular immunoglobulin (Ig)-like domains and three cytoplasmic tyrosine-based signaling motifs, one of which is included in a classical immunoreceptor tyrosine-based inhibitory motif. Engagement of NTB-A on human T cells can substitute the CD28 co-stimulatory pathway and induces polarization toward a Th1 phenotype. However, CD4-positive T cells from NTB-A (Ly-108) knockout mice show impairment in IL-4 production, suggesting a role of NTB-A in Th2 polarization. The reason for this discrepancy is not fully elucidated. Activation of NTB-A on human NK cells stimulates cytotoxicity and proliferation, as well as IFN-γ and TNF-α production.
Valdez et al, 2004 teach that NTB-A activates T cells by homotypic interactions, and specifically enhances Th1 properties. An NTB-A-Fc fusion protein, produced by fusing the first 226 amino acids of NTB-A to the Fc portion of murine IgG1, was found to inhibit B cell isotype switching, commonly induced by Th1-type cytokines, and inhibited a Th1-dependent autoimmune disease (EAE model). Thus, the reported NTB-A fusion protein was found to act as an NTB-A antagonist in the experimental systems reported by Valdez et al.
US 2009/017014 to Valdez et al is directed to the PRO20080 polypeptide (having an amino acid sequence corresponding to that of NTB-A), the extracellular portion thereof, homologs, agonists and antagonists thereof, which are suggested as putative modulators of immune diseases. The '014 publication suggests the use of certain immunostimulating compounds disclosed therein in immunoadjuvant therapy for the treatment of cancer.
Uzana et al., 2012 disclose that NTB-A blockade on antigen presenting cells (APC) by specific antibodies inhibited cytokine secretion from CD8+ lymphocytes. While the publication suggests this molecule as a potential target for improving anti-cancer immunotherapy, experimental exploration of the relevance of this approach is said to be warranted, since similar approaches, targeting other co-stimulatory receptors such as CD28 with agonistic antibodies, ended up in a fatal outcome in clinical trials.
Since NTB-A is expressed on certain hematopoietic tumors, vaccination using peptide epitopes derived from this molecule has been proposed, to induce an anti-tumor immune response against tumors aberrantly expressing this antigen. For example, PCT Pub. No. WO 2006/037421 discloses 338 peptide sequences derived from HLA class II molecules of human tumor cell lines, which can be used in vaccine compositions for eliciting anti-tumor immune responses. Among these sequences is a 16 amino acid peptide corresponding to positions 103-118 of SLAMF6. In addition, targeting these epitopes with antibodies or immunotoxin conjugates thereof has been suggested. For instance, US2011171204 discloses anti-NTB-A antibodies and antigen-binding fragments thereof, and methods of using the same to bind NTB-A and treat diseases, such as hematologic malignancies, which are characterized by expression of NTB-A. Additional antibodies against NTB-A are described, for example, by Krover et al., 2007. These antibodies exerted cytotoxic effects on NTB-A expressing lymphocytes, and had no effect on T cell proliferation or cytokine secretion.
EP2083088 discloses a method for treating cancer in a patient comprising modulating the level of an expression product of a gene selected from the group consisting of inter alia SLAMF6, wherein the cancer is selected from the group consisting of melanoma, breast cancer, colon cancer, kidney cancer, liver cancer, lung cancer, ovary cancer, pancreatic cancer, prostate cancer, uterine cancer, cervical cancer, bladder cancer, stomach cancer or skin cancer. The publication discloses that the method is useful for treating a patient characterized by over-expression of said gene.
WO 03/008449 relates to NTB-A polypeptides, nucleic acid molecules encoding the same and uses thereof. The publication also relates to methods of regulating NK cells activity by regulating the activity of NTB-A in vitro, ex vivo or in vivo, and to methods of screening active compounds using NTB-A or fragments thereof, or nucleic acid encoding the same, or recombinant host cells expressing said polypeptide. Further disclosed is the use of a compound that regulates the activity of a NTB-A polypeptide in the preparation of a medicament to regulate an immune function in a subject.
X-linked lymphoproliferative disease (XLP) is a rare congenital immunodeficiency that leads to an extreme, usually fatal increase in the number of lymphocytes upon infection with Epstein-Barr virus (EBV). XLP commonly results from deficiency of SLAM-associated protein (SAP), also designated XLP1, caused by mutations in the SH2D1A gene on chromosome Xq24-25. Accordingly, NTB-A has been implicated in this pathology, as contributing to the inability of NK-cells to kill EBV-infected B-cells.
Snow et al. (2009, 2010) examined the role of NTB-A and its downstream effector SAP, in the regulation of restimulation-induced cell death (RICD) of T cells obtained from healthy donors and XLP patients. The publications report that in normal donor T cells, NTB-A is positively involved in, and necessary for, TCR-induced apoptosis. In contrast, in XLP patients this phenomenon is reversed, as NTB-A was found to contribute to RICD resistance in XLP T cells.
Interleukin-2 is a critical growth factor for lymphocytes in culture. In its absence, activated T cells cannot be maintained and are subjected to activation induced cell death. Thus, IL-2 is widely used in cell culture, for both experimental purposes and clinical applications, in the preparation of various cell compositions, vaccines and immunotherapies for cancer, autoimmune diseases, and other immune mediated pathologies.
IL-2 is also used as a therapeutic agent in vivo. For example, IL-2 is licensed by the Food and Drug Administration (FDA) for the treatment of kidney cancer, and its significant therapeutic effects on patients have been reported in many clinical trials. In clinical trials with patients with melanoma, IL-2 increased the survival rates of many individuals, which were higher than the average survival rates usually obtained with chemotherapy drugs.
However, it has been repeatedly shown that the use of IL-2 enhances the appearance of regulatory T cells, which may eventually exert an inhibitory effect. This issue is of particular relevance when using T cells in the clinical context, both in vivo for the systemic treatment of patients, and ex vivo, for the production of lymphocytes for adoptive cell therapy. A satisfactory substitution for IL-2 has not been found yet.
In addition, a major difficulty in using exogenous IL-2 and other exogenous cytokines in vivo is their high toxicity. Side effects are of considerable magnitude, particularly cardiac symptoms and dysfunctions, septic shock and fever. This requires intensive care for adverse effects remediation or control, and may also lead to discontinuation of treatment.
There remains an unmet medical need for improved therapeutic modalities for the management of cancer. In addition, further means for enhancing the propagation and activation of T cells in culture and for providing improved cell vaccine compositions are required. Safer, more effective therapies, circumventing the drawbacks of currently used treatment, would thus be desired.